In the past years the lithographic technology applied to transferring patterns from masks to devices under construction has become highly developed and widely used. Masks, such as those employed in these processes, are used with a variety of radiation sources such as light, both visible and ultraviolet, as well as X-rays and electron beams. An example of an electron beam system is given in Broers et al, U.S. Pat. No. 3,876,883. Other systems employing light as a radiation source are found in U.S. Pat. Nos. 3,152,938; 3,458,370; 3,712,816; 3,758,326 and 3,832,176.
Masks for use with electron beam projection processing apparatus, such as that disclosed in the aforementioned Broers et al patent have a number of requirements in common with other masks, and some requirements which are unique to electron beam masks. In the first place, one desires the ability to provide a small aperture which is well defined. The desire for this characteristic should be apparent to those skilled in the art in that the minimum size opening in the mask limits the size of the features that can be transferred from the mask to any receiver (although not necessarily on a one-to-one basis since some exposure systems may actually reduce the mask features during exposure.) The ability to transfer a particular pattern is based upon the mask definition of that pattern. In addition to the foregoing requirements, however, a requirement unique to electron beam projection masks is the requirement that the mask be self-supporting, even if the mask is heated during its operation by absorption of electrons. For example, in some applications the mask must remain plane to within 1 mm across a 3 inch span without any external support and in the presence of heat. Since the electron beam is substantially incapable of passing through even the thinnest of substrates, the mask must be characterized by the absence of material at locations where it is desired that the electron beam pass through the mask. Since the mask must have sufficient strength and rigidity to remain in an integral condition without outside support, the mask itself must be thick enough to provide this function. To provide self-support and yet be capable of very high resolution patterns, the mask thickness often must exceed the lateral dimension of the smallest aperture in the mask. That is, the ratio of thickness to lateral dimension (or aspect ratio) must be greater than 1. In addition, since most, if not all, electron beam projection systems employ magnetic focusing coils to project a pattern smaller by a factor as much as ten times smaller than the original pattern one desires the mask to be comprised of non-magnetic material so as to not interfere with the focusing operation. Since electrons impinging onto the mask dissipate heat a further requirement is that the mask be constructed of metal capable of high heat conductivity to permit fast enough removal of heat and prevent overheating of the mask. Masks produced by this technique can also be used as X-ray masks in X-ray lithography.
In addition, since such masks are preferably made by additive plating techniques, they should be preferably constructed from readily platable metals such as gold, copper, silver, platinum, chrome, zine or any other such non-magnetic readily platable metal or alloy thereof.
Although the prior art evidences methods of forming masks and screens a problem exists in providing a method of forming a mask which is thick enough to be self-supporting, particularly when heated as by impact of electrons and when as large as about 3 inches in diameter which is capable of providing a very small aperture, which is well defined and with aspect ratio greater than 1. The prior art processes are sufficient to provide small apertures reasonably well defined by employing a computer controlled mask or the like and exposing an electron beam, photoresist or other radiation sensitive material to radiation which is modulated by a desired pattern. A single such exposure will not, however, in accordance with the prior art teachings, provide a sufficiently thick mask to be self-supporting. If one attempts to increase the thickness by merely extending the parameters of the prior art processes using a single deposited layer definition is degraded. In particular the radius of curvature of corners increases with increase in resist thickness. When the radius increases above certain limits definition is degraded. On the other hand, it is possible to employ a plurality of exposures of a photoresist or other suitable radiation sensitive material with the well known steps of developing and/or developing and plating between exposures. One difficulty with this approach is that the masks employed for the several exposures must be aligned with the partially formed mask in each of the different exposures. Due to the extremely small dimensions of the various pattern components on the mask this has proved to be a time consuming operation.
One particular way of using such electron beam masks imposes further restrictions on mask tolerances. Specifically, in some applications the mask aperture negative tolerance is zero. That is, the mask aperture must be no smaller than the nominal size. Clearly if one employs a method of forming such mask with multiple steps of coating, exposure, development and plating with identical forming masks, there is absolutely no tolerance, for even the smallest misalignment will reduce the resulting aperture below nominal size. This aspect imposes almost insurmountable problems to the multiple step operation employing identical masks.
It is therefore an object of the present invention to provide a method of fabricating a mask suitable for electron beam processes which eliminates the necessity for this precise alignment in several different exposures. It is another object of the present invention to provide a method for fabricating a mask suitable for electron beam process which is self-supporting and yet which eliminates the necessity for practically unattainable mask alignment during fabrication of the mask itself.
Furthermore, it is a purpose of this invention to provide masks made of readily platable non-magnetic materials with good heat conductivity to permit quick removal of heat which accumulates due to electrons striking the mask and dissipating heat.